1. Field of the Invention
The present invention relates to an active matrix liquid crystal light valve (AMLCV) for switching a liquid crystal cell by an active element thereof, to a liquid crystal display apparatus (LCD) having the light valve, and to an image information processing apparatus having the LCD.
2. Related Background Art
Hitherto, a liquid crystal display (LCD) having an active element has been, as an AMLCV, widely used in a structure which comprises twisted nematic (TN) liquid crystal, and have been marketed as a flat panel display or a projection TV monitor. The active element typified by a thin film transistor (TFT), or a diode or a MIM (Metal Insulator Metal Element) enhances the optical switch response of TN liquid crystal which suffers from relatively slow response, by keeping a state, in which the TN liquid crystal is being applied with voltage, for a period longer than the actual line selection period. Furthermore, the active element causes liquid crystal device such as the TN liquid crystal having no memory characteristics (self-holding characteristics) to have a substantial memory state for each unit cell for one frame by keeping the aforesaid voltage applied state. The LCD has excellent display characteristics because it is theoretically freed from crosstalk between lines and between pixels thereof.
Recently, ferroelectric liquid crystal (FLC) revealing the response speed higher than that of the TN liquid crystal by a degree of several digits has been developed energetically, resulting in display panels and light valves using the same to be disclosed. In the aforesaid circumstance, there is a possibility that a further excellent display device can be obtained by driving the FLC by the active matrix device. As an example structured by combining the FLC and the TFT has been disclosed in, for example, U.S. Pat. No. 4,840,426 and in Proceeding of the SID, vol. 30, 1989 "Ferroelectric Liquid-Crystal Video Display" vol. 30, 1989.
FIG. 11 illustrates a conventional liquid crystal display circuit.
The circuit shown in FIG. 11 comprises a unit pixel composed of a common electrode COM, a liquid crystal cell 701 filled with liquid crystal material between its pixel electrodes CE, and a pixel TFT 702. The circuit still further comprises a signal line 703, a line buffer 704, a shift pulse switch 708, and a horizontal shift register 705 for transmitting video signals. The circuit further comprises a gate line 711 and a vertical shift register 706 for transmitting gate signals. The video signals are received by a signal input terminal 707 so as to be sequentially transferred to each pixel or each line while having their timing shifted.
FIG. 12 illustrates the timing of drive pulses for use in the conventional active matrix liquid crystal display device shown in FIG. 11. FIG. 12 illustrates the timing of the drive pulses for use in a line sequential drive method. That is, video signal Sv to be recorded on the liquid crystal is recorded in such a manner that video signals for one line are recorded on the buffer portion via a shift pulse switch 708 which is operated by the horizontal shift register 705 arranged to transmit an output in synchronization with the frequency of the video signal Sv. After the video signals for all of the pixels for one line have been recorded to the line buffer portion 704, the video signal is recorded to each liquid crystal cell via a pixel switch, which has been switched on by an output switch of the line buffer portion 704 and the vertical shift register 706. The signals are usually transferred to each liquid crystal cell during a blanking period of a horizontal scanning period or transferred collectively to a certain horizontal line in response to pulse .phi.t. At the aforesaid timing, the video signals are sequentially transferred to each line.
When molecules of the liquid crystal, which forms the cell, move in accordance with the voltages of the signals thus transferred, the transmittance of the liquid crystal cell is changed in accordance with the direction of the deflection plate individually provided to have a relationship of a cross polarizer. The aforesaid state is shown in FIG. 13.
The voltage of the signal shown in the axis of abscissa of FIG. 13 is meant different facts depending upon the type of liquid crystal. For example, the values are defined to be effective voltage values (Vrms) in the case of the TN liquid crystal. The qualitative description of the aforesaid value will be made with reference to FIG. 14. In order to prevent a fact that DC components are applied to the liquid crystal for a long time, there is a method in which the polarity of the signal voltage is altered for each frame at the time of supplying the signal. In this case, the liquid crystal acts in accordance with the AC voltage component shown by a portion designated by a diagonal line. Therefore, execution voltage V.sub.rms is expressed as follows when the time for two frames is t.sub.F and the signal voltage to be transferred to the liquid crystal is V.sub.LC (t): ##EQU1##
On the other hand, the FLC is ordinarily driven by DC voltage. In a case where FLC of a type having a bistable state is employed (it is preferable that chiral smectic liquid crystal be used, further preferable chiral smectic liquid crystal such as phase C (SmC*), phase H (SmH*), SmI*, SmF* or SmG* chiral smectic liquid crystal be used), drive waveforms shown in FIG. 15 are offered. That is, the signal voltage is reset to either of the bistable states in accordance with reset voltage V.sub.R before the signal is written, and then writing voltage signal (V.sub.M) is applied. Also the signal voltage contributing to the transmittance shown in FIG. 13 is designated by diagonal lines. In a manner different from the TN liquid crystal, the DC component of the writing voltage is the signal voltage as it is.
Although the voltage of the pixel electrode is changed in accordance with the signal voltage if the drive method shown in FIG. 12 is used, it is always positive with respect to the potential of the common electrode of the liquid crystal similarly to the case where a DC voltage component is always applied to the liquid crystal cell. In the case where the TN liquid crystal is used as the liquid crystal material, the aforesaid DC component causes a problem to arise in that the liquid crystal molecules can be burnt.
Methods of removing the DC voltage component is typified by a method of reversing the signal voltage for each frame arranged as shown in FIG. 14. The signal voltage at the N-th time is so applied as to be positive with respect to the potential of the common electrode, while the signal voltage at the (N+1)-th time is so applied as to be negative. By reversing the polarity of the signal voltage with respect to the potential of the common electrode for each frame as described above, the DC voltage components to be applied to the liquid crystal cell are set off so that burning of the liquid crystal molecules can be prevented.
Similarly, a method of reversing the same for each 1H and a method of reversing the same for each pixel may be employed. However, the aforesaid reversing drive method arises the following problems.
Assuming that the maximum value of the signal voltage is V.sub.MAX, the shift register portion must have, regardless of the type of the reversing method, performance capable of transferring a signal having an amplitude which is two times the VMAX if the reversing drive method is employed. Therefore, the shift register portion must, of course, be able to withstand the ON/OFF voltage.
As a means for relaxing the required condition about the voltage resistance, it might be feasible to employ a method in which the maximum amplitude of the signal voltage is reduced. However, the aforesaid means cannot be preferably adapted to a high vision display which is expected to be rapidly widely used in the future and which must have excellent precision because it is difficult to keep the gradation as can be understood from FIG. 13.
Another method can be employed in which a voltage-resisting MOS transistor such as a LDD (Lightly Doped Drain) serving as a switch is used as a transistor which constitutes the shift register. However, the aforesaid voltage-resisting MOS transistor, which is being developed currently, arises a problem in that the mutual conductance (gm) is lowered due to enlargement of the resistance, which is in series applied to the source and the drain although it is able to improve the voltage resistance. As described above, the LCV must be, as an active device, driven at further high speed as in the case of the high vision display. Therefore, the TFT must have a larger gm. What is worse, the MOS transistor having the voltage resistance as described above can be manufactured only from a complicated process, causing problems to arise in that the yield deteriorates when it is used to constitute the shift resistor and that the manufacturing cost cannot be reduced.
There is another desire of improving the drive speed in addition to the aforesaid desire of improving the voltage resistance in order to display an excellent image. In particular, it is necessary to raise the speed of writing data to the line buffer in the case where the line sequential drive method is employed. Although it might be considered to raise the frequency of the shift pulse in order to achieve this, the circuit shown in FIG. 11 and the drive method shown in FIG. 12 cannot satisfactorily raise the aforesaid frequency.
In a circumstance in which the quantity of information is enlarged, the aforesaid problem causes software to bear a larger load or causes hardware such as the memory quantity and the microprocessor (MPU) to bear a larger load.